Measurement Methods And Sources Of Error; Useful Information - Megger MFT1800 Series User Manual

Residual Current Device (RCD) or Ground Fault Current Interrupt (GFCI) testing
If using the 1/2xI, 2xl or 5xI test ranges, the touch voltage will still be displayed for 1xI test current,
as per IEC 61557-6.
2xl and 5xI can create real touch voltages during the tests that are higher than the displayed value. If this voltage
exceeds the touch voltage limit (UL) the test will be stopped.
Under these conditions the display will show the calculated touch voltage on the small digital segments and >50 V on
the larger digital segments, as below:
X
8.15

Measurement methods and sources of error

RCD Testing - Method of measurement
A two wire lead, or mains plug lead should be used for this measurement. A constant current source is connected
across the supply and the time taken for the supply to trip is measured by the instrument in ms.
RCD Testing – Possible sources of error
Measurement results can be affected by the following:
„ Significant operating errors can occur if loads, particularly rotating machinery and capacitive loads, are left
connected during tests.
A poor connection to the circuit under test.
„
8.16

Useful information

It is only necessary to test the 10 mA and 30 mA at 1/2xI, 1xI and 5xI. All other RCDs only need to be tested at 1/2xI
and 1xI.
Always press the RCD TEST button on the RCD to ensure the function works.
It is recommended the RCD test button is tested AFTER the timing tests above are complete. This can identify RCDs
that may stick or fail if not checked periodically.
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